Various machine structural steel parts used in industrial machinery, construction machinery, conveying machinery such as automobiles, and the like are often produced by roughly working a steel for machine structural use to a predetermined form or shape by hot working, such as hot forging, and then finishing the same to a desired form or shape by machining. Accordingly, the steel for machine structural use has been required to have not only good mechanical properties but also high machinability.
With the advances in automated high-speed machining steps in recent years, the demand for structural steels excellent in machinability, in particular structural steels, not only excellent in chip separability but also enabling carbide tools used in machining them to secure a long tool life, has been increasing for stably realizing safety and high productivity.
According to the prior art, Pb (lead) is added to improve the separability of chips of steels for machine structural use. In view of the recent increasing concern about environmental problems, however, structural steels showing good chip separability without addition of Pb are desired.
Well known Pb-free structural steels, having machinability when subjected to machining with carbide tools, are calciumized free cutting steels. In calciumized free cutting steels, low-melting-point oxides are formed and these protect the carbide tools and prolong the tool life.
However, as described in DENKI-SEIKO (ELECTRIC FURNACE STEEL), Vol. 44, No. 1, pp. 81 to 88, for instance, calciumized free cutting steels are poor in chip separability as compared with leaded free cutting steels. Therefore, the combined use of a chip separability-increasing element, such as S (sulfur), is necessary, and calciumized and resulfurized free cutting steels have generally been used. In the case of calciumized and resulfurized free cutting steels, however, oxide morphology control is carried out and, accordingly, the substantial oxygen content increases and coarse sulfides are formed in some instances, leading to failure to secure good chip separability. Thus, it is difficult to stably increase the chip separability of Pb-free steels.
In laid-open Japanese Patent Application (JP-A) No. H11-222646, a structural steel excellent in chip separability is disclosed which has a substantially Pb-free composition and is characterized in that there exist individual sulfide inclusions not shorter than 20 μm, or groups of a plurality of sulfide inclusions linked together in an approximately linear manner and not shorter than 20 μm in a section in the direction of rolling in a density of 30 or more per square millimeter. However, for producing this steel, it is necessary to modify not only the steelmaking conditions but also the rolling conditions, and this technology is therefore under severe restrictions.
JP-A No. 2000-219936 proposes a free cutting steel having a specified composition and characterized in that it contains 5 or more sulfide inclusions, containing 0.1 to 10% of calcium and having a circle equivalent diameter of 5 μm or larger per 3.3 square millimeters. However, since the aim of the invention disclosed in this publication was to improve the material anisotropy and tool life by dispersing sulfide inclusions containing not more than 10% of CaS in the MnS, no attention has been paid to the improvement in chip separability.
JP-A No. 2000-282171 discloses a structural steel excellent in chip separability and characterized in that it has a substantially Pb-free composition and also has a sulfide grain distributing index of not more than 0.5. However, calculations, made by the present inventors, of the sulfide grain distribution indices, as proposed in the above-cited publication, for the common steels grade S1 and grade S2 improved in machinability, as described in the Japanese Automobile Standards Organization standard JASO M 106-92 (established May 28, 1977 and revised Mar. 30, 1992 by the Society of Automotive Engineers of Japan), failed to find such steel species that have the desired mechanical characteristics and machinability under which the index in question has a value of 0.5 or lower.
JP-A No. S57-140853 discloses a “calciumized and resulfurized free cutting steel, restricted in soluble Al content to 0.002 to 0.005% by weight and in O (oxygen) content to 0.0040% by weight or less, and containing not more than 0.0150% by weight of Ca within the range of (Ca %−0.7×O %)/S %≧0.10 (% being % by weight)”. This calciumized and resulfurized free cutting steel indeed makes it possible to accomplish the purposes of preventing sulfide extension and securing low-melting-point oxides simultaneously and, therefore, is effective in improving the tool life. However, when the Ca content is high and exceeds 0.01%, coarse sulfide inclusions may be formed and, therefore, good chip separability cannot always be obtained simultaneously.
Japanese Patent Publication (JP-B) No. H05-15777 discloses a “calciumized and resulfurized free cutting steel containing 0.015 to 0.060% by weight of Al with the O (oxygen) content being 20 ppm or less” for deoxidation and grain size control. The calciumized and resulfurized free cutting steel proposed in this publication is indeed improved in chip separability as compared with S-containing free cutting steels and Ca-containing oxide controlled steels, but from the chip separability viewpoint, it is still inferior to Pb-containing free cutting steels.